ACCRA (Reuters) - West African leaders will seek authority next week from the African Union to create a multi-national force to fight Nigeria's Boko Haram Islamist insurgents, Ghanaian President John Mahama told Reuters on Friday.

Any such force would represent the most robust international response yet to the militants who have killed thousands over the last year in their campaign for an Islamic caliphate and have also launched cross border attacks into Niger and Cameroon.

Boko Haram is seen as the most serious security threat to Nigeria, Africa's most populous country and its biggest energy producer, but Mahama said the group and militants in Somalia, Kenya, Mali and elsewhere posed a wider risk.

"Terrorism is like a cancer and if we don't deal with it will keep going. It threatens everybody in the sub region. When it comes to terrorism nobody is too far or too near," he said.

It will take months before an African Union force could be set up and key issues such as who would command it, the location of its headquarters and its financing remain undecided, he said.

Once set up, however, the African Union could ultimately seek a United Nations Security Council mandate to take over the force as happened in Sudan's Darfur region, he said.

Mahama was speaking as current chair of West African regional bloc ECOWAS, which has been accused of not doing enough to combat Boko Haram.

"Nigeria is taking military action and Cameroon is fighting Boko Haram, but I think we are increasingly getting to the point where probably a regional or a multinational force is coming into consideration," he said earlier.

In a further blow, Boko Haram militants seized the military base and town of Baga, in Nigeria on the shores of Lake Chad, on Jan. 3. Baga was the headquarters of a planned force to fight the insurgents with troops from Nigeria, Chad, Niger and Cameroon, although that initiative stalled.

The insurgency stirred international outrage when militants seized more than 200 school girls in Chibok, northeastern Nigeria, the epicenter of the violence, last April.

France must do more to help countries fight Boko Haram, President Francois Hollande told an annual conference of French and foreign ambassadors in Paris.

"Today, Cameroon, Niger, Chad and Benin are threatened and this situation means the international community must take appropriate action and can't let this be," he said.

France said last month it would help coordinate a regional task force against Boko Haram given signs of mistrust among West African neighbors.

Cameroon President Paul Biya this month appealed for military help against Boko Haram. On Friday, U.S. Ambassador Michael Stephen Hoza said Washington would help train Cameroon's soldiers and offered equipment for the fight.